For the purpose of this description, the term "honing" and the term "superfinishing" or "super-finish grinding" may be used interchangeably to indicate a fine grinding or finishing operation in which a surface of a workpiece is abraded to leave the high quality, usually bright metal surface practically free from surface irregularities. While technically superfinishing and honing may not be exactly equivalent because various movements of the workpiece and tool can be involved in honing while others may be involved in super-finish grinding, for the purposes of the present invention, the honing, superfinishing and super-finish grinding processes will be considered to be equivalent, to preferably use a cup-shaped tool whose rim is active to abrade a surface of the workpiece and which is set in rotation while the workpiece is rotated or other means is provided for generating relative displacement of the tool and workpiece.
Generally in such treatments of a workpiece surface to finish this surface, a coolant is supplied to the interface between the workpiece and the tool to carry away any machining detritus and to cool the workpiece surface which might otherwise deform because of the intensity of heating resulting from the rubbing action of the tool against the workpiece.
The tool may be a so-called diamond wheel.
In practice, the coolant is a grinding oil or a grinding emulsion and is applied in a fine stream or jet with relatively low pressure directly to the machining region, i.e. the jet is trained upon the region at which the tool contacts the workpiece surface.
It has been found that this does not prevent distortion of the workpiece as a result of uncontrolled heating of the latter since, especially when the workpiece has a thin wall, heat conductivity between the area in direct contact with the tool and the adjacent surfaces of the workpiece bathed in the cooling liquid, is comparatively poor. As a result, local overheating can occur. Local overheating can result in distortion of the workpiece and excessive machining and material removal at certain locations.
Experience has shown that in spite of the use of large quantities of cooling liquid in these earlier cases, temperatures up to 800.degree. C. can be detected in the region in which the tool abrades the workpiece. Since the tool covers the workpiece in these regions, it can frequently prevent access of the coolant thereto and hence it is preferred to use a narrow surface of the tool so as to minimize the area over which excessive heating may occur.
This is not a complete answer to the problem since nevertheless excess of heating may occur in the contact region and the smaller area of contact may create other problems with respect to uniformity in precision and flatness of the machining operation.
The problem is especially pronounced for so-called composite workpieces where elements thereof may have different thermal coefficients of expansion and contraction. The same applies for workpieces which because of their configuration, e.g. because of a reduction in cross section, are incapable of providing an adequate heat conductivity away from the machining site.
With all of these workpieces, even in relatively short machining times of 5 to 10 seconds, thermal deformation can and frequently does occur under conventional conditions and the thermal deformation may remain upon cooling. In practice it turns out to be difficult, if not impossible, to hold the planarity, sphericity or like tolerances to 1-3 microns as is frequently necessary.